Consequences of Cloud Water Deposition on Vegetation at High Elevation

Abstract

The capture of wind-driven cloud by vegetation provides a pathway for pollutant deposition that has only recently been identified, and remains only poorly quantified. This paper reviews current knowledge of three aspects of the pathway: measurement and modelling the rates of deposition of cloud water to various Vegetation types; techniques for monitoring the concentrations of soluble and solid material in cloudwater; and potential mechanisms for injury to Vegetation by the deposited material.

Although there have been many measurements of amount of fog drip below trees, there are very few where there is sufficient detail of the environmental conditions and plant structure to allow generalizations to be drawn. Analysis of existing measurements Supports the view that fog-water fluxes are essentially limited only by rates of turbulent transfer and so can be modelled realistically from a knowledge of momentum transfer. This can be shown to imply that fog-water fluxes are typically 1 mg.m⁻².s⁻¹ (4 μm.h⁻¹) over Short grass and 10 mg.m⁻².s⁻¹ (40 μm.h⁻¹) over forests. Analysis of drag forces on leaves and shoots can be used to show that isolated trees and shrubs capture fog water at rates of up to 100 mg.m⁻².s⁻¹, consistent with the few useful observations.

These analyses suggest that there is substantial spatial variation in fog-water deposition, for example at the upwind edges of forests, and on dominant trees in canopies. There will also be large vertical variation in a dense canopy in the amount of water deposited per unit foliage density — current knowledge of in-canopy wind profiles can be used to estimate this. Chemical concentrations in cloud water can be much larger than in rain. Apart from data at Whiteface Mountain, there do not appear to be long series of records to establish the “climatology” of cloud-water chemistry, i.e., its dependence on synoptic meteorology and the seasonal variability at sites. The design of collectors probably leads to biases, because of differing variations of collection efficiencies with drop size. In Britain, cloud water with large concentrations of acidic substances often contains substantial amounts of solid particles, and so vegetation may have much larger inputs of particulate material in this occult deposition than we have previously estimated when considering only dry deposition.

The low pH of cloud water may, by itself, be sufficient to damage foliage, by eroding cuticular waxes and, for example, directly injuring epidermal cells, or allowing increased leaching, or altering gas or water vapour exchange. Evaporation from the water film on leaf surfaces, proceeding simultaneously with evaporation, may lead to a further concentration of solution on the surface. Evaporation from forests would be very sensitive to small alterations in humidity (in the range 97–100% rh). For extensive forest canopies, the concentration ratio (surface liquid/cloud drop) may exceed 10, leading to surface pH below 3. The spatial variation of the concentration phenomenon within canopies and on isolated trees is likely to be large, and could help to explain the positioning of observed visible injury.

Large inputs of sulphur and nitrogen to upland forests by occult deposition may also increase the probability of frost injury to trees, both directly by altering membrane strength and indirectly through changes in rates of cold hardening.